1. Field of the Invention
Exemplary embodiments of the present invention relate to an apparatus and method for forming a thin layer on a substrate. More particularly, exemplary embodiments of the present invention relate to an apparatus and method for forming a thin layer on a semiconductor substrate by increasing pressure of a source gas.
2. Description of the Related Art
A thin layer is formed on a semiconductor substrate, such as a wafer, and processed into a pattern including electrical characteristics to fabricate a semiconductor device. The thin layer is formed on the substrate by a deposition process, such as a chemical vapor deposition (CVD) process, a physical vapor deposition (PVD) process and an atomic layer deposition (ALD) process. As recent semiconductor devices have high degrees of integration and small unit cells with reduced pattern line widths, the ALD process has become widely used as the deposition process for forming the thin layer.
A deposition apparatus for the ALD process (hereinafter referred to as an ALD apparatus) is classified as either a horizontal type or a vertical type in accordance with a shape and orientation of a processing chamber. The vertical type ALD apparatus has been the most widely used because the footprint of the vertical type ALD apparatus is much smaller than that of the horizontal type ALD apparatus. The vertical type ALD apparatus includes a vertical processing chamber that includes an inner space defined by vertical walls. A wafer is placed into the inner space of the vertical processing chamber, which provides a vacuum environment. A number of wafers are placed into the inner space of the vertical processing chamber at one time for improving the deposition efficiency of the vertical ALD apparatus using a batch process.
In general, source materials for a thin layer must be supplied in a gaseous state into a processing chamber. Thus, the vertical type ALD apparatus has a liquid delivery system which includes a vaporizer for vaporizing liquid source materials and an injector for uniformly supplying the vaporized source materials into the processing chamber. More particularly, in the case of a batch-type ALD apparatus, vertically uniform injection of the vaporized source materials is essential for the uniformity of the thin layers because multiple wafers are vertically stacked in the processing chamber.
The higher the degree of integration of a semiconductor device, the smaller the thickness of the thin layer. The thin layer in a highly integrated semiconductor device therefore includes dielectric materials having a high dielectric constant. However, the material having the high dielectric constant usually has a high molecular weight, and is not sufficiently vaporized in the vaporizer due to the high molecular weight. Therefore, the material having the high dielectric constant is supplied to the injector at a relatively low pressure. As a result, a relatively large amount of the source material is supplied into the processing chamber through the injector near the vaporizer, and a relatively small amount of the source material is supplied into the processing chamber through the injector far from the vaporizer.
Further, the vaporizer generally requires a constant temperature for efficient maintenance. An idling process is therefore performed in the vaporizer even when the processing chamber stops performing the ALD process. During the idling process, for example, an inert gas, such as nitrogen gas, is injected into the processing chamber and exhausted from the processing chamber through an exhaust pump. Accordingly, the idling process is necessarily performed in a preliminary operation period during which the source materials are vaporized for the ALD process, and thus the reaction gas for the ALD process does not have sufficient pressure when supplied from the injector. Therefore, much more of a reaction gas is supplied into the processing chamber through the injector nearest the vaporizer, and much less of a reaction gas is supplied into the processing chamber through the injector furthest from the vaporizer.
Due to the above-described structural features of the conventional ALD apparatus, the reaction gas does not have sufficient pressure when injected into the processing chamber through the injector, and is non-uniformly supplied into the processing chamber. The non-uniform supply of the reaction gas leads to non-uniformity in the thickness of the thin layer on a wafer and causes various processing defects in subsequent processes on the wafer.
FIG. 1 is a scanning electron microscope (SEM) picture showing a hafnium oxide (HfO2) layer formed by an ALD process in a conventional batch-type ALD apparatus. FIG. 2 is a view illustrating a thickness distribution of a thin layer formed by a conventional batch-type ALD apparatus. The hafnium oxide layer in FIG. 1 was formed on a wafer of about 200 mm, and the thin layer in FIG. 2 was formed on a wafer of about 300 mm. Both of the wafers in FIGS. 1 and 2 were positioned at a top portion of a wafer boat extended vertically in a vertical processing chamber.
Referring to FIG. 1, a peripheral portion 12 of the hafnium oxide layer is thicker than a central portion 14, creating an appearance similar to a volcano crater. The hafnium oxide layer was formed into an average thickness of about 31.49 Å with a deviation of about 2.12 Å. Experimental results show that an average deviation of all the wafers in the vertical wafer boat was about 3.09 Å. Similarly, FIG. 2 shows that the thin layer has a thickness of about 20.76 Å at a central portion of the wafer and gradually increases toward a peripheral portion.
The thickness reduction of the thin layer at the central portion of the wafer causes a current leakage from the central portion. The current leakage deteriorates the performance of semiconductor devices.
Proposals have been suggested for improving the flow uniformity of the reaction gas when the reaction gas is supplied into the processing chamber in the conventional batch-type ALD apparatus. For example, the injector has been modified to include several injection holes having sizes gradually vary in accordance with a vertical direction of the processing chamber, or the vaporized source gas was more highly pressurized. Further, it has been suggested that a buffer be installed around the injector and thus allow the reaction gas to flow into the processing chamber at a steady-state steady-flow (SSSF). However, these proposals have not sufficiently improved the flow uniformity of the reaction gas. Moreover, the proposals generally require significant additional costs, thereby reducing the manufacturing efficiency of semiconductor devices.